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Oct- 2, 1962
c. w. MUSSER
3,056,311
ROTARY~LINEAR TRANSDUCERS
Filed NOV. 2, 1961
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‘wave .LEacn-H m WAVE GENERATOR?
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Inventor
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3,056,311v
Patented Oct. 2, 1962
2
1
are caused to have a component of movement radially
ROTARY-LINEAR TRANSDUCERS
C. Walton Mussel‘, Beverly, Mass, assignor to United
Shoe Machinery Corporation, Boston, Mass, a corpo
ration of New Jersey
Filed Nov. 2, 1961, Ser. No. 149,542
4 Claims. (Cl. 74--424.8)
inwardly or outwardly.
The above and other features of the invention, includ
ing details of construction, will now be more particular
ly described in connection with an illustrative embodi
ment, and with reference to the accompanying drawings
thereof, in which:
the nut and the screw member, one or more tubular
‘FIG. 6 is a section taken on the line V~I—VI of FIG. 2
3,056,311
FIG. 1 is an axial section showing a transducer com
prising an elliptoidal screw, a nut, and intermediate roll
This invention relates to mechanism for e?iciently con
verting rotary to linear movement and vice-versa. More 10 ing elements;
FIG. 2 is a slightly diagrammatic section taken on the
speci?cally, the invention pertains to a transducer assem
line II-~II of FIG. 1;
bly of the general type sometimes referred to as a ball
FIG. 3 is a similar section taken on the line III-1H
bearing screw.
of FIG. 1 and, when considered with FIG. 2, illustrates
In the usual ball bearing screw arrangement relative
motion of parts is obtained with rolling contact between 15 phase relationship of adjacent raceways;
FIG. 4 is a view indicating how one form of the tooth
them instead of sliding contact thus providing easier op
or thread of the screw is developed;
eration. Customarily such construction has comprised a
FlG. 5 is a section taken on the line V--V of FIG. 2,
plurality of rolling elements or balls in helical grooves
and illustrating the relation of the rolling elements with
formed in a nut and in matching helical grooves on a
the screw at the larger axis of one raceway or groove
cooperating screw member within the nut. For recircu
and at the minor axis of the adjacent raceways or
lating the rolling elements from one end of their helical
grooves;
travel to the starting points of their movement between
and illustrating the relationship of the rolling elements
guides across the outside of the nut has normally been
required. Although the transducer of this invention 25 of the screw at a position 90° from that shown in FIG. 5;
FIG. 7 is a diagrammatic view showing a preferred
super?cially appears similar to prior constructions in
dimensional relationship of the parts with reference to
that it includes a screw member and a nut cooperating
therewith through rolling elements, the assemblage of
peak~to-peak amplitude;
provide a device of the type referred to wherein no addi
therewith. These balls are preferably all alike as to size.
FIG. 8 is a diagrammatic view of the elliptoidal shape
this invention is clearly distinguished both in the manner
of cooperation of its parts, and in their number and 30 of a raceway together with the mathematical expression
therefor; and
con?guration.
FIG. 9 is a development of the raceway amplitude
Thus, for instance, as herein shown in contrast, the
vdepicted in FIG. 8.
rolling elements do not individually ?t the groove con
"Referring to FIGS. 1 to 3, a lobar screw 20, elliptoidal
volutions of the screw, but remain in one raceway in the
in cross section, is formed with multiple threads 22 (FIG.
nut; recirculating guides are not needed; the raceways in
1), in this case two, for cooperating with corresponding
the nut do not have ‘a helix angle; and a plurality of roll
‘axially spaced, elliptoidal sets of raceways 24, 26 formed
ing element raceways in the nut cooperate with a single
internally of a nut 28. It will be understood that if, for
thread in the screw.
instance, the screw were provided with a triple lead, each
A primary object of my invention is to provide im
proved means of the ball bearing screw type for convert 4.0 of the nut raceways or grooves 24, 26 would be formed
with a 3-wave or tri~lobar shape instead of being ellip
ing rotary to linear movement or linear to rotary move
toidal. Each thread 22 as illustrated in FIG. 1 has a
ment, in which friction between the parts is further re
plurality of rolling elements such as balls 3%, 32 in contact
duced and ef?ciency increased. Further objects are to
tional means is required for recirculating the rolling ele 45 While the thread 22 has suitable pitch, the raceways 24, 26
have no pitch but are, ‘as shown in FIGS. 2 and 3, 90°
ments, a relatively high axial travel of an output mem
out of phase with each other.
ber per revolution of the nut is obtainable, and the nut
In FIG. 1 four similar sets of raceways 24, 26 are illus
may in comparison with prior construction, have a rela
trated, each set comprising grooves spaced one-half pitch
tively smaller over-all diameter with respect to that of a
50 apart; i.e. they maintain the centers of the balls 30, 32
screw.
spaced apart axially by a distance equal to one-half the
‘In accordance with these objects, a feature of the in
distance between adjacent peaks of the screw threads.
vention resides in the provision of transducer mechanism
The relationship of the adjacent raceways 24, 26, as seen
comprising a screw lobar in cross section and having at
from a comparison of FIGS. 2 and 3, further is that they
least one thread of selected pitch, a relatively rotatable
nut in telescoping relation on the screw, the nut having 55 constitute elliptoids which are 90° out of phase with
each other about the axis of rotation of the assembly.
a plurality of raceways correspondingly lobar but of zero
Hence, all of the rolling elements 30, 32 are radially mov
pitch, and a plurality of roller elements in each raceway,
able during operation of the assembly and those shown
the amplitude of the raceway corresponding to that of
in FIG. 1 are midway of their radial displacement. The
the screw to maintain the elements in contact with both
the nut and the thread during relative movement. As 60 arrangement is such that, if the nut 28 is rotated relative
ly to the screw 2%, all of the balls shown in FIG. 1 would
herein illustrated, the screw (sometimes termed a wave
generator) and the raceways are elliptoidal in cross sec
tion but it is to be understood that they may have a trans
verse con?guration providing more than two lobes re
spectively and accordingly the term “lobar” is .employed
with respect to the screw and raceway to comprehend all
of such shapes. Indeed, a device could come within the
scope of the invention if the raceways of the nut were
circular but eccentric to each other. The basic principle
to be noted, however, is that as the screw and nut are
relatively rotated, the rolling elements in the raceways
be moving radially, half of them being moved outwardly
and half of them inwardly. ‘Consequently the inwardly
moving balls would act on the threads 22 to cam the
65 screw axially. Incidentally, it will be recognized that
FIGS. 2 ‘and 3 are not strictly true sections in that, in fact,
the balls 30, 32 are not in contact with the screw 20 as
shown except at the peaks and valleys of the threads.
‘FIG. 4 illustrates a development of a thread 22. Its
contour is that of a sine wave modi?ed as required in
order to accommodate the spherical elements 30, 32.
Various diameters for these elements may be used, and
3,056,311
3
4
also dilferent pitch can be selected for a given transducer
constructed in accordance with this invention. For the
conditions illustrated in FIG. 4 a sine wave form having
the axis of rotation, it will be understood that this is
not mandatory so long as their actual spacing is appropri
ately selected to accord with the speci?c phase relation
ship. As shown the raceways 24, 26 are separated by
one-half pitch and the elliptoids are separated by 90°. If
a peak-to-peak magnitude designated a’ is shown, the pitch
and the roller diameter likewise being indicated in terms
of d. The axial pitch was selected to provide the maxi
a different pitch were selected so that it would not crowd
mum angle for the sine wave of 45 °, thus yielding a
pressure angle on the screw 20 which will be about 45°.
the raceways, they could be separated diiferently. For
instance, the raceways could be separated one—third pitch
This is to say that, for these conditions, the balls will be
and the elliptoids would then be out of phase by 60° in~
functioning against the Wall of the thread 22 at this angle. 10 stead of 90°. It will further be apparent that no speci?c
The path of the centers of each of the balls 30‘, 32 is
or strictly sine wave shape need be used for the screw
shown as a sine wave 34 (FIG. 4) having a peak-to-peak
thread or the nut raceways.
amplitude d, and a maximum slope of 45°, the axial pitch
As above indicated a simple transducer of the general
p then equaling the dimension 1r times d. Travel of the
type described, but not herein shown, could be provided
ball centers along the wave 34 generates a wave shape 15 by a screw having a single lead thread. Circular race
which is the contour of the threads 22.
ways in the nut would then be utilized, the cooperating
As shown herein the axial pitch p is the axial distance
nut raceways being eccentric. If in this case the raceways
from one thread to the next thread of the screw 20. The
were placed one-half pitch apart axially, their eccentricity
wave form 34, therefore, in addition to representing the
would be 180° out of phase. Operation of this device
path of the centers of the rolling elements in FIG. 4,
would be similar to that previously described. The cir
also represents the sine Wave of the elliptoidal raceways
cular nut raceway eccentric to the axis of the screw would,
24, '26 in the nut 28 with which the balls 30, 32 must
upon rotation, produce the illustrated sine wave and it
always be in contact. The sine wave 34, when referred
would have a peak-to-peak amplitude dependent on the
to the elliptoidal screw 20, is accordingly a wave length
degree of eccentricity.
for one-half revolution of the screw. This is illustrated in 25
While the balls 30, 32 have been herein shown in con
FIGS. 8 and 9 wherein the axial sine wave 34 is desig
tact with each other, it will be appreciated that any
nated 36 in the elliptoidal con?guration. The sine wave
suitable separators may be employed if desired.
36 should have the same peak-to-peak amplitude d as the
Having thus described my invention, what I claim as
sine Wave 34 to maintain all the balls 30, 32 in continual
new and desire to secure by Letters Patent of the United
contact with the thread 22 and the raceways 24, 26 30 States is:
through their entire surfaces.
l. A transducer comprising a transversely lobar screw
FIG. 5 illustrates the relationship of the rolling ele
having at least one thread of selected pitch, a relatively
ments 30, 32 with the thread 22 at that location wherein
rotatable nut in telescoping relation on the screw, the nut
an element 30 is at the major axis of the raceway 26, ‘and
having a plurality of raceways correspondingly lobar in
at either side an element 32 is at the minor axis of their 35
cross section but of zero pitch, and a plurality of roller
respective raceways 24. These elements, in these extreme
positions of their radial movement, cannot be useful in
elements in each raceway, the peak-to-peak amplitude of
the raceways corresponding to that of the thread to main
tain said elements in contact with both the nut and the
generating axial motion of the screw 20 since they cannot
serve as cams While at the peak and the valley of the wave.
Of the other roller elements otherwise situated in the re 40 screw during their relative movement.
2. A transducer according to claim 1 and further char
spective raceways 24, 26 one-half are moving radially
acterized in that the screw is formed with a pair of ex
inward to contribute to producing relative axial motion in
ternal threads having the axial shape of a sine wave,
one direction, and the ‘other half are rolling outwardly
the raceways of the nut are elliptoidal in con?guration, one
on the slope on the opposite side of the thread and thus
tending to prevent overtravel of the screw. Those balls 45 pair of raceways cooperating with each thread and being
out of phase with each other, and the roller elements are
30, ‘32 at the cross-over point of the wave which are on
balls of a diameter greater than the depth of said wave.
the 45° slope ‘contribute most to the conversion from
3. A transducer according to claim 1 wherein the
rotary to linear movement. FIG. 6 shows the relationship
balls in the several raceways are of the same diameter and
of the roller elements and the screw thread at the location
wherein an element 30 is von the minor axis of the race
way '26 and adjacent elements 32 are now at the major
50
adjacent raceways of the nut are 90° out of phase.
4. A transducer comprising a screw of elliptoidal cross
section and having a pair of threads of selected pitch,
‘axis of the respective raceways 24.
a relatively rotatable nut in telescoping relation on the
FIG. 7 shows diagrammatically a preferred dimensional
screw, the nut having a plurality of pairs of elliptoidal
relationship of the parts, the diameter of the balls being
somewhat greater than the height d of the sine wave. 55 raceways of zero pitch, the raceways of each pair being
spaced one-half pitch apart for cooperating with one of
Protrusion of the roller element from the thread 22
said threads and the axial pitch of each thread being equal
should always occur to some extent so that the element
to 1r times the peak-to-peak amplitude of the raceways,
can be engaged by the nut 28. It is also necessary for
and a plurality of roller elements in the latter having a
the latter to have clearance with the screw 20. Thus
the minimum depth of a nut raceway may be as shown 60 diameter, respectively, at least as great as said amplitude.
at 38 (FIG. 7) and its maximum depth may be as in
dicated at 40, the maximum depth groove being illustrat
ed by the dotted line 42. The di?erence in the maximum
‘and minimum raceway depths is of course equal to the
sine wave height d.
It will be apparent that the number of sets of raceway
grooves for a given nut 28 may be selected according
References Cited in the ?le of this patent
UNITED STATES PATENTS
2,616,302
2,739,491
2,924,113
Wahlmark ____________ __ Nov. 4, 1952
Russell ______________ .._ Mar. 27, 1956
Orner ________________ __ Feb. 9, 1960
348,847
Germany ____________ -._ Feb. 17, 1922
FOREIGN PATENTS
to the desired load-carrying capacity. While the adjacent
‘grooves have herein been shown 90° out of phase about